In manufacturing various components such as components for automobiles, components for construction machines and the like, with the aim of imparting cold workability to hot rolled material such as carbon steel, alloy steel and the like, cold working is executed after spheroidizing annealing treatment is executed, thereafter cutting work and the like is executed for forming into a predetermined shape, quenching and tempering treatment is thereafter executed for final strength adjustment.
In recent years, there is a trend that the shape of the components becomes more complicated and larger, and, accompanying that, in cold working step, there is a request to further soften steel, to prevent cracking of steel, and to improve the life of a die. In order to further soften steel, although execution of spheroidizing annealing treatment for a longer time is also one method, from the viewpoint of energy saving, there is a problem in extending the heat treatment time excessively.
Several proposals have been also made so far with respect to steel for promoting spheroidizing. For example, in Patent Literature 1, it is disclosed that a steel wire rod containing pro-eutectoid ferrite and pearlite with the average grain size of 6-15 μm and with the volume percentage of pro-eutectoid ferrite being in a predetermined range can achieve both of quick spheroidizing annealing treatment and cold forgeability. However, when the microstructure is miniaturized, although the time for spheroidizing annealing treatment can be shortened, softening of the material when ordinary spheroidizing annealing treatment of approximately 10-30 hours is executed is insufficient.
On the other hand, in Patent Literature 2, a technology is disclosed in which softening is achieved as hot rolled by specifying the size of the dislocation cell and the grain size number of ferrite. However, this technology is also still insufficient in terms of further softening.